Switch matrixes are ubiquitous in signal transmission systems. Generally, switch matrixes serve to direct one of various inputs to a desired output. Of particular interest herein are switch matrixes used in high frequency applications such as those involving microwave and other radio frequency (RF) signals (collectively referred to herein as “RF signal applications”). Examples of RF signal applications using switch matrixes include, for example, instrumentation such as radar, and telecommunications such as cell phones.
An essential component of a signal switch matrix is a cross connect (or combiner) which serves to couple each of the inputs with each of the outputs. These are generally passive devices employing “combiners” to combine signals from “n” number of input conductors on to “m” number of output conductors, resulting in an n×m switch matrix. This way, a signal on each and every input conductor is coupled to each and every output conductor. Examples of cross connects include n×m perfect shuffles in which n=m, or asymmetrical cross connects in which n≠m, with typically n>m. As used herein, the term “cross connect” refers collectively to cross connects, combiners, and splitters having the configuration described above.
In the electronics field, the need to decrease costs and increase performance creates a constant need and desire to miniaturize. There is particular pressure to reduce the “foot print” of switch matrixes to facilitate their installation in densely-packed electronic devices such as aircraft radar systems, hand-held cellular phones, and other miniature communication devices. A promising solution to reduce the foot print is the use of multilayer circuit boards in these devices. Multilayer circuit boards have become popular recently as a way of concentrating electronics and freeing critical surface area for other components. Basically, a multi-layer circuit board comprises a number of circuit boards in which each circuit board comprises circuitry interconnected by transition vias running perpendicularly through the boards.
Although multi-layer circuit boards provide an attractive solution for minimizing the foot print of signal switch matrixes, they tend to lack sufficient isolation between channels. Isolation within the switch matrix is critical to ensure that signals on one channel are not corrupted by those on another. Isolation becomes more difficult as the frequency of the signals increases. For example, isolation is particularly difficult at the operating frequencies contemplated in this application—that is, in 20 GHz range. The difficulty in isolating high frequency signals is exasperated by the miniaturization of these components. As mentioned above, there is significant pressure to reduce the size of switching circuits. This reduction in size necessarily requires that the channels be more densely packed which makes them more susceptible to interference. Further exasperating the problem is the fact that most of the channels in a switch matrix tend to be amplified, and, therefore, any errant RF mode induced on the channels will be amplified.
Therefore, there is a need for a switch matrix system that is compact but maintains good isolation between the signal channels. The present invention fulfills this need among others.